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Artificial Hearts

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A thumb-sized heart pump

Scientists at Oxford are working on a very sophisticated project to develop a thumb-sized heart which in the future may offer 30 years of extra life.

This small heart can be fitted into the chest and starts working at the first sign of heart failure.

Results from early trials using animals indicate that the pump is effective. The first implants for humans are expected to be available within five years. The pump fits directly into the left ventricle, helping to propel each heart beat's surge of freshly oxygenated blood into the aorta, the massive blood vessel which supplies the entire body.

The plastic pump can provide up to 16 pints of blood a minute if required.

Mechanical circulatory support

References:
Braunwald, Heart disease textbook 5th ed.

Medical therapy is the initial treatment for heart failure. Unfortunately it is not always effective, and patients must be maintained on mechanical circulatory support (bridge to transplantation) until they can be helped by heart transplantation, which is currently the most effective treatment for heart failure.

The hemodynamic criteria for using mechanical circulatory support - relevant when adequate pre load, maximal pharmacological support, corrected metabolism, and intra aortic balloon pumping for more than one week have already been tried - are listed below:

  1. cardiac index <1.8 litres/m2/min
  2. systolic arterial pressure <90mm Hg
  3. left and/or right atrial pressure >20mmHg
  4. urine output <20ml/hr (adult)
  5. systemic vascular resistance >2100 dynes
  6. metabolic acidosis

A number of mechanical devices are now available, which can be classified in several ways, such as internal or external, electrical or pneumatically powered, but in general we can divide them into two groups: non pusatile and pulsatile.

  • Non pulsatile:
    • Hemopump
    • Roller pump
    • Centrifugal pump
  • Pulsatile:
    • ECMO
    • Novacor
    • Heart mate
    • Symbion AVAD
    • Symbion TAH
    • A biomed

External centrifugal and roller pumps

These pumps are positioned extracorporeally and can be connected to cannulae (plastic tubes) that are placed in any of the heart chambers to provide the desired type of support. These are the least expensive mechanical devices, but patient mobility is usually impaired because of their size and external position. These pumps are best suited to a short duration of support (less than one week); however, there have been some cases of support for longer than 30 days.

External pulsatile ventricular assist devices

These have had fairly extensive worldwide use. The Pierce-Donachy Thoratic VAD has been used successfully in both bridge to transplantation and post cardiotomy cardiogenic shock patients.

The Symbion device, known as the AVAD (acute ventricular assist device), is similar to one ventricle of the Symbion J-7 artificial heart. The devices are powered by compressed air, they all contain mechanical valves, and they can provide biventricular support for long time (up to 200 days).

The A biomed device (BVS 5000) significantly impairs patients mobility.

Implantable left ventricular assist system

Implantable left ventricular assist systems are particularly important since these devices are prototypes of permanent systems which will provide an alternative to heart transplantation for patients with end-stage heart disease. Two devices currently being used in clinical studies are Novacor LVAS, and Heart mate LVAS. These devices, however, may not be well suited for postcardiotomy use, as they are designed for left ventricular cannulation only and require removal of a portion of ventricular tissue at insertion.

Novacor is an electrical system available now, and the external power cable is the only element traversing the skin, allowing for excellent patient mobility. It can serve for up to 400 days or more.

Heart mate is similar to the Novacor, except that it uses compressed air as its power source. It has a unique feature in that it uses a textured rather than a smooth blood-contacting surface which promotes the development of a viable biological lining of the blood, and it does not require anticoagulation.

Bi ventricular Replacement Prostheses

These artificial hearts are perhaps the most publicized devices available; the Symbion Jarvik-7, T.A.H has been used worldwide. It is pneumatically powered and requires removal of the patient's ventricles. It has had limited use as a permanent implant and has been used since the mid-1980's as a bridge to transplantation, but some centres are developing permanent artificial hearts.

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